FIG. 1 shows a conventional sense circuit 100 configured with a balance circuit for high-side sensing, which comprises two resistors R1 and R2 connected to a sensed object 106 at the opposite sides, a transistor 107 having a source 108 connected to the resistor R1 and a drain 110 and gate 112 connected to a current source 102, a transistor 113 having a source 114 connected to the resistor R2, a gate 118 connected to the gate 112 of the transistor 107, and a drain 116 connected to a current source 104, a transistor 119 having a source 120 connected to the source 108 of the transistor 107, a gate 124 connected to the drain 116 of the transistor 113, and a drain 122 connected to a resistor Ro.
Based on the voltages at the opposite sides of the sensed object 106, two currents I1 and I2 flow through the resistors R1 and R2, respectively, and each of the currents provided by the current sources 102 and 104 is equal to the current I2. If the current I1 is larger than the current I2, the difference Io between the currents I1 and I2 will flow through the transistor 119 and resistor Ro to produce an output voltage Vo, by which the voltage drop ΔV across the sensed object 106 is determined.
FIG. 2 shows a conventional sense circuit 200 configured with a balance circuit for a comparator, in which the balance circuit is the same as that of the sense circuit 100, and an output voltage Vo is generated on the drain 116 of the transistor 113 in response to the difference between the input voltages VII and VNI connected to the resistors R1 and R2.
However, the real devices for the transistors 107 and 113 will not match to each other, and there is always an offset current present during the sensing carried out by the sense circuit 100 or 200. In other words, the sense circuits 100 and 200 never implement precise sensing. Though tuner circuit may be introduced to eliminate the influence caused by the offset current, proper tuning could not be achieved for sense circuits of mass production, since the sense circuits of mass production have not uniform offset currents.
Therefore, it is desired an offset independent sense circuit and method.